HYALONEMA (HYALONEMA) AGASSIZI. 185 



In the spicule-preparations of the basal part of this form numerous small, 

 hollow, cross-Uke siUceous bodies were observed. The smallest of these are about 

 20 fi in diameter, and consist of four somewhat conic rays, 10 m long, about IG m 

 thick at the base, and hoUowed out by cylindrical axial canals about 8 ^ wide. 

 These smallest crosses are connected with the large normal stout-rayed tetractines 

 above described by an uninterrupted series of spicules intermediate in size. 

 The axial canals of these spicules are usually 5-9 m wide. The axes of the rays 

 of the full-sized, stout-rayed basal spicules are occupied either by a fine axial 

 thread, or a more or less widened axial canal. The broadest axial canals in these 

 spicules were about 9 m in diameter. In cylindrical, terminally rounded rays 

 these axial canals are closed at the end ; in conical and pointed rays they usually 

 open out freely. 



The wide axial canals are regular or irregular. The regular ones are either 

 cylindrical throughout or widened distally. Distal widenings occur both in the 

 terminally open and in the terminally closed axial canals. The irregular ones 

 are of two kinds. In some the axial canals bear short, irregular, branch-Uke 

 diverticula, which usually arise near the distal end, and are vertical or oblique, 

 directed outward or, more rarely, inward. Others possess backwardly directed 

 diverticula, which arise from their basal part and occupy interstices between 

 adjacent silica-layers. 



It is obvious that the small forms of this series are to be considered either 

 as the yoimg of the full-sized ones, or as the last remnants of full-sized ones which 

 have in great part been dissolved. The general appearance of the whole series 

 seems to me to be in favour of the latter alternative. I accordingly assume that 

 the stout acanthophores with wide axial canals are spicules in process of decay 

 (solution), that this decay or solution is the further advanced the smaller the 

 spicules are, and that the dissolving agency acts on the silica-layer both from 

 the inner (the axial threads) and the outer side (the surface). No doubt the sea- 

 water can and does dissolve the silica of the spicule in this way when the protect- 

 ing organic or semiorganic sheath is lost, but it must not be overlooked that the 

 living sponge-tissue of the sponge itself might possibly also attack and dissolve 

 the silica in spicules which have become superfluous, and use the material thus 

 obtained for building up other spicules. 



The spheres of form A (Plate 45, figs. 26-34) are irregularly nodular or 

 spherical and measure 18-57 /u in diameter. Most of them are smooth (Plate 45, 

 figs. 26-29, 33, 34), some more or less spiny (Plate 45, figs. 30-32). They consist 

 of concentric layers of silica. The centre around which these silica-layers are 



